1. Field of the Invention
The present invention relates to a spatial coherent light transmission apparatus for use in an optical communication system or the like.
2. Description of the Related Art
The spatial data transmission using light, which is free from indoor interference unlike a transmission using an electric wave as a medium, has been conventionally used for an information transmission for a short distance.
This transmission system has employed an intensity modified detection (IMDD) method in which a light emission diode (LED) is used as a transmission light source and the modulation of the light intensity thereof allows data to be decoded. On the other hand, a spatial light transmission apparatus using a coherent detection technique has been put into practical use.
In these days, data transmission such as digital moving image transmission is required to perform the transmission with a higher speed. However, data transmission for a long distance has been impossible because the optical current power decreases in proportion to the communication distance to the fourth power.
For example, the transmission side uses an LED having an emission power of 100 mW diffusing light over .+-.60 degrees for a spatial transmission while the receiving side has a light receiving portion in the shape of a circle with a diameter of 1 cm. In this case, the communication distance, which allows the transmission speed to be 100 Mbit/s and the transmission bit error rate (BER) to be 10.sup.-9, with the use of a baseband modulation, is 1 m or less. Hereinafter, it is assumed that the BER is less than 10.sup.-9 where a signal to noise ratio (SNR) is 20 dB.
The conventional spatial transmission apparatus using a coherent light detection will be described.
FIG. 1 is a diagram showing the configuration of a beat signal detection device for use in a conventional wireless optical communication system. As shown in FIG. 1, a signal beam 1 is incident through an incidence window portion 2 having a collecting lens. Then, the signal beam 1 passes through an optical multiplexer/demultiplexer 3 and is incident onto one of light receiving elements 4a of a light receiving element array 4. On the other hand, a locally oscillated beam is generated from a semiconductor laser 5 and is collimated by a collimator lens 6. Thereafter, the locally oscillated beam is diffused by a diffusion optical element 7 and becomes a diffused locally oscillated beam 8, which is diffusion plane waves. The travelling direction of the locally oscillated diffused beam 8 is changed by an optical multiplexer/demultiplexer 3. Thereafter, the diffused beam 8 is mixed with the signal beam 1 and is incident onto the light receiving element array 4. Herein, a selfog lens array (intervals between lenses: 1 mm, and numerical aperture: 0.2) is used as the diffusion optical element 7. By means of this diffusion optical element 7, the plane wave components of a diffused locally oscillated beam 8a at intervals of 0.05 degree exist over the light receiving array 4. The angular range where the components exist is determined by the numerical aperture of the diffusion optical element 7, which is a selfog lens array, and is .+-.11.5 degrees. An angular range larger than .+-.11.5 degrees weakens each of the plane wave components to a degree such that it makes actual communication impossible. Herein, if the deviation of the incident direction of the signal beam 1 is within .+-.11.5 degrees, a beat signal can be detected by the diffused locally oscillated beam 8a. However, in the case where the incident direction deviates out of this range, the optical multiplexer/demultiplexer 3 is rotated by an actuator so that the beat signal can be detected.
This actuator will be described hereinafter. The optical multiplexer/demultiplexer 3 is disposed on a rotation stage 10 to which a magnet 9 is attached. The central portion of the rotation stage 10 is adhered to a rod with a soft resin adhesive (not shown). Furthermore, coils 11 and 12 generating magnetic fields in vertical and horizontal directions with respect to the paper surface of FIG. 1 are respectively provided so that the rotation stage 10 and the optical multiplexer/demultiplexer 3 can be rotated by means of the interaction between the magnetic fields and the magnet 9.
The signal detected by the light receiving element array 4 is processed by a control circuit 13 so as to optimize currents to flow into current sources 14 and 15. Thus, coherent detection can be performed by making the wavefront of the signal beam 1 and that of the diffused locally oscillated beam 8a coincide with each other (wavefront alignment). Also, the detected beat signal is processed by a beat signal frequency control circuit 16 so that the wavelengths of the signal been 1 and the locally oscillated been 8a are controlled with a predetermined relationship therebetween. For this purpose, the temperature of the semiconductor laser 5 serving as an LD generating the locally oscillated beam is controlled by a temperature controller 17 (wavelength tuning).
Such a conventional configuration has an advantage that a data transmission at a high speed is normally assured, but has problems as follows:
1. Under the condition where the beat signal is not detected e.g., at the time of starting the communication, a long time has to be taken for scanning the temperature or the like to realize wavelength tuning, which hinders the communication being performed at a high speed;
2. If the beat signal cannot be detected because a deviation arises either in the wavefront alignment and wavelength tuning at the time of starting the communication or during data transmission, it is impossible to judge whether the wavefront or wavelength deviates. As a result, a long time is taken before cancelling the deviation. This is a great problem, especially in the case where the apparatus is mounted on a moving object, because wavefront deviation is always likely to occur.
3. If any problem occurs during data transmission, such as deviation of wavefront or wavelength, an obstacle existing on the optical path, etc., data is lost over the corresponding term. Moreover, the transmission side cannot detect the condition of the receiving side, so that the timing of transmitting data, the confirmation whether or not the data is received, the certification of the transmitted data and the like have been impossible.
4. In a short distance communication, the data cannot be stably received, since coherent detection between a spherical wave (signal beam) and a plane wave (locally oscillated beam) is employed. This problem will be described in more detail hereinafter.
Now, it is assumed that a light receiving element with a circular light receiving portion occupies a circular shape on the x-y plane, where the center of the circle is it the origin and the radius thereof is R. When the transmission light source is disposed at the point (0, 0, z) on the z axis, a photocurrent I (z, r), excited at the point apart from the origin by a distance of r, is expressed by the following Equation (1): ##EQU1## where P.sub.1 and P.sub.2 are power densities of the transmission light source and the local oscillation light source, h.omega. is an energy of light, e is an elementary electric charge, .di-elect cons. is a power transmittance of the signal beam, .eta. is a photon-electron conversion efficiency, and .lambda. is a central wavelength of the light. Besides, .DELTA..omega. and .DELTA..phi. are differences in angular frequency end light phase between the transmission beam and the locally oscillated beam, and the third term of this equation stands for a beat signal.
Herein, a current flowing through the entire light receiving element, I.sub.Total (z), is obtained by integrating this photocurrent I (z, r) over a light receiving face as expressed by the following Equation (2): ##EQU2## where an approximate value expressed by the following Expression (3) is used: ##EQU3##
As is apparent from the above equations, the value of the amplitude of a beam signal current monitored over the entire light receiving element oscillates depending on the distance z. Accordingly, when the distance z is large, the light receiving portion has to become large in size in order to receive and demodulate a weak signal beam. On the other hand, when the distance z is small, reliable receiving operation is impossible, because the amplitude of the beat signal is unstable.
FIG. 2 shows characteristics of the receiving power and the noise power in the IMDD method of 100 MHz. Herein, the power of the signal beam light source is 100 mW, a light emission angle is .+-.60 degrees, the radius of the light receiving element is 5 mm and the receiving frequency hand width is 100 MHz. FIG. 3 shows the characteristics of the signal power and the noise power in the coherent detection of 100 Mbit/s. Herein, the transmission light source is a semiconductor laser diode (LD) adjusted so as to have a power of 100 mW, an amplitude of 10 MHz, and a light emission angle of .+-.60 degrees. The light receiving element has a radius of 2 mm, and the receiving frequency band width is 100 MHz. In each of the figures, .oval-hollow. denotes the demodulated signal power and .oval-solid. denotes the noise power.
As is apparent from these figures, in the case of IMDD, although a transmission with a high SRN is assured in a short distance communication, the SRN sharply deteriorates in a long distance communication. In the case of coherent detection, though an SNR higher than that of the IMDD method can be obtained in a long distance communication, a stable receiving operation is not assured in a short distance communication.
Furthermore, Japanese Laid-Open Patent Publication No. 3-46839 proposes a coherent light transmission between satellites. According to this proposal, a light receiver detects the fluctuation in the beat signal frequency corresponding to the amount of change in the wavelength caused by a Doppler shift. The information on the fluctuation is sent to a transmitter as an electric wave or a light signal. On the transmission side, the fluctuation amount is absorbed, whereby the transmission wavelength is roughly adjusted.
This proposed configuration allows a stable wavelength tuning in the case where no deviation arises in the wavelength for communication between facing stations. However, in spatial coherent transmission with the use of a household mobile communication apparatus or the like, the interruption of communication due to wavefront deviation or an obstacle existing in the course of transmission is problematic. The loss of data cannot be prevented in the configuration. Also, in this configuration, the transmission wavelength on the transmission side fluctuates. As a result, it is impossible to provide two or more receiving side devices with respect to one transmission side.